What is urine?

The actual amount depends on how much you eat, drink and sweat,
and whether you're taking medication such as diuretics (water tablets).

Your urinary system

The ureters

Urine drains from your kidneys to your bladder through tubes
called ureters.

Your bladder

Your bladder is the hollow muscular organ that stores your
urine.

It’s a bit like a floppy balloon – as your bladder fills, it
expands to store the urine.

On average, your bladder can hold 400 to 600ml of urine for up
to about five hours.

The muscle in your bladder wall is called the detrusor muscle.
The detrusor muscle relaxes to allow your bladder to fill. When you go to the
toilet, it contracts to squeeze out urine.

Your urethra

Did you know?

The female urethra is shorter than the male urethra.

This is why women are more likely to get urine infections
than men.

At the bottom of your bladder is the urethra. This is a tube
about 4cm long that runs from the neck of your bladder to the outside of your
body, just in front of your vagina.

The urethra has a valve known as the urinary
sphincter.

Your sphincters

There are two sphincter mechanisms.

The internal sphincter is a ring of muscle
that holds the neck of the bladder in place. Your body opens and shuts it
automatically without you thinking about it.

The external sphincter acts like a tap and
keeps urine in the bladder. It is controlled by the pudendal nerve, which is
controlled by the voluntary nervous system. This means it's under your control
– you decide when to let it open. The external sphincter is also called the
distal sphincter.

Your pelvic floor

Below your bladder there is a hammock of muscles known as the
pelvic floor that supports the bladder, bowels and the womb.

At the side it's connected to your pelvic bone.

At the front it’s connected to your pubic bone.

At the back it's connected to your tailbone or
coccyx.

The pelvic floor also:

keeps your urethra closed until you're ready to pass
water

squeezes when you cough or sneeze, to help stop urine
leaking

controls the expulsion of wind

increases your sexual awareness during intercourse.

How do you know when to empty your bladder?

The micturition reflex tells you when you need to empty your
bladder. This happens on average four to eight times a day. The reflex is
controlled by your central nervous system.

When your bladder is about half full, the stretch receptors in
the walls of your bladder become active and send signals along your pelvic
nerves to your spinal cord.

A reflex signal is sent back to your bladder, which makes the
detrusor muscle in the bladder wall contract. The contraction increases the
pressure in your bladder, and this is what makes you want to pass urine.

Because the external sphincter is under voluntary control, you
don’t urinate until you decide to relax this muscle.

What happens when you urinate

Your brain sends a series of messages to enable your body to
pass urine:

to your detrusor muscle in the bladder wall, telling it to
contract and squeeze the urine out into the urethra

to your sphincter, telling it to relax and open

to your pelvic floor, telling it to relax and allow the
sphincter to open.

3. The nerves that communicate with your brain and spinal cord

The pelvic floor, the bladder and SUI

If your pelvic floor is weak, it isn’t strong enough to hold
your bladder up properly. The base of your bladder may then protrude through
your pelvic floor.

If this happens, whenever there’s any stress on the pelvic floor
(for example when you cough, sneeze, laugh or lift something heavy), the
muscles can't cope with the extra pressure and allow a small amount of urine to
leak through the urethra.

One way to visualise how it works is as if you're holding a
balloon full of liquid, with the neck of the balloon between your finger and
thumb.

The balloon is the bladder, the neck the uretha, your finger and
thumb the pelvic floor muscles.

When your finger and thumb get tired, you don't hold the balloon
so tightly. If someone presses down on the balloon, liquid will leak
out.

Prolapse

Pelvic floor weakness also makes prolapse more likely. This
means SUI and prolapse often happen together.

A prolapse is the protrusion of a pelvic organ into the
vagina:

the bladder can prolapse causing a bulge in the front wall of
the vagina

the bowel can prolapse causing a bulge in the back wall of
the vagina

the womb can prolapse into the vagina from above and can
sometimes be visible at the entrance to the vagina.

If you have SUI, your doctor will examine you carefully to
assess whether you have a prolapse (and vice versa).

The materials in this web site are in no way intended to replace the professional medical care, advice, diagnosis or treatment of a doctor. The web site does not have answers to all problems. Answers to specific problems may not apply to everyone. If you notice medical symptoms or feel ill, you should consult your doctor - for further information see our Terms and conditions.